The present invention relates to heated containers for retaining a material in a molten state.
Tar, asphalt and like materials are often utilized for roadway pot hole repairs and for other pavement maintenance work, being easily applied in a molten state. Tar pots for maintaining the material in a molten or liquid condition commonly comprise a single shell container to which heat is directly applied in order to heat and melt the tar material therewithin. Inasmuch as the tar is often initially placed into the tar pot in a solid mass for liquification, the heating of the container or tank is generally performed at relatively high temperatures to effect the softening of the tar within an accelerated period of time.
Typically the material is heated by directing a flame from an acetylene torch or propane gas burner or the like onto a particular spot or spots along the outer surface of the container shell. At those spots or areas upon which the flame impinges the shell is highly heated by the flame and this heat is transferred by conduction throughout the remainder of the container shell. Conduction heat losses in the shell, however, cause local temperatures at the points of direct heating to substantially exceed the temperature at locations removed therefrom. The tar adjacent the shell is thus heated through contact therewith and the temperature of the remainder of the tar is elevated as a result of heat transfer throughout the material. It is, of course, recognized that the temperature distribution throughout the tank will be highly non-uniform since the tar immediately adjacent directly heated portions of the shell will be significantly hotter than the material adjacent the remainder of the shell and material located interior of the bounds or walls of the container.
It is well know that tar breaks down or "cokes" under relatively low temperature heating in the region of 550.degree. or 575.degree. F., the decomposition resulting in the formation of hardened chunks of carbonaceous material. The tar is thereby rendered useless for its usual applications as it loses its elasticity and cannot be reliquified by further heating. The initial addition to the tar of foreign substances to enable the tarring material to perform more fully the functions that may be needed--such as in filling cracks in roadways and the like--results in a lowering of the decomposition or "coking" temperature of the tar to approximately 450.degree. F.
As a result, the tar immediately adjacent the heated container shell upon which the flame is directed cooks and breaks down along the interior surface of the container, depositing thereon a carbonaceous residue. These deposits are poor heat conductors and consequently reduce the efficiency of the heat transfer which is relied upon to maintain the bulk of the tar material in the interior of the tar pot in a molten state. The use of high heat thus not only renders unusable at least a portion of the tar material heated but, in addition, significantly reduces the efficiency of the heating operation, necessitating the use of additional fuel to attain and maintain the desired heated condition. The direct impingement of the flame onto the outer surface of the container also serves to fairly rapidly erode and deteriorate the shell. necessitating its frequent replacement.
Recognizing the propensity of the tar material to coke at such relatively low temperatures. Various devices for heating the material has been proposed wherein the flame does not directly impinge upon the container. In U.S. Pat. Nos. 3,315,659 to Schmitz and 3,503,382 to Wollner, a gas burner is utilized to project heated air into heat conducting pipes or ducts immersibly disposed within the tar-holding chamber for maintaining the tar at elevated temperatures. The lack of any disclosed regulating mechanism capable of limiting the temperature of the heated air projected into the pipes or ducts suggests that decomposition of the tar will still occur adjacent thereto. In addition, the piping will clearly be hotter at its connection with the source of heated air than at the exhaust end, resulting in an uneven temperature distribution throughout the piping and therefore within the tar material.
Attempts have also been made in enclosing or encasing the tar material within a multi-walled structure. Such multi-walled structures may include an outer shell and an inner shell between which a fluid such as oil or air may be circulated. When the fluid is heated, it is hoped that the same will impart a more evenly distributed elevated temperature to the tar-holding inner shell to thereby avoid the possibility of coking. U.S. Pat. No. 2,728,336 to Elgeti utilizes a gas burner to project heated air between inner and outer shells. Insulatively disposed sand and granite is provided to prevent excess dissipation of heat from the outer shell. Again, however, the absence of any ability to adequately insure that temperatures will be limited to maintain the inner shell below the decomposition temperature of the tar suggest a high probability of coking about the inner shell. This problem is further compounded by the provision of insulative materials to keep excess heat between the two shells which will have the effect of enabling large amounts of heat to build up within the enclosure thereby further raising the temperature of the inner shell.
Thagard, Jr., in U.S. Pat. No. 4,028,527, teaches the use of thermostatically-controlled electric heating elements embedded within or otherwise in direct communication with the container shell. While this system will apparently provide a fairly constant and regulated shell temperature, the necessity of insuring access to a source of electric potential for operation of the heating elements severely limits the portability of such a unit in a relatively small-scale field use, such as the filling of pot holes or related pavement repair work. In addition, the high cost of electricity renders operation of the Thagard, Jr. apparatus extremely and possibly prohibitively expensive for such typical tar-related repair work.
It is, therefore, the desideratum of the present invention to provide an apparatus for heating a material-holding container to a substantially constant selected temperature below the decomposition temperature of the material by utilizing a source of heat emitting radiation at a relatively high temperature above the decomposition temperature of the material.